The present invention relates to an offshore transport vessel unloading system. Transferring fluids, particularly cryogenic fluids, on the open ocean in unprotected locations offers particular hazards in terms of personnel safety and damage to the vessels or facilities involved the fluids which are transported in a transport vessel from a remote location may be delivered to either a tank in the offshore location, or by pipeline to a land-based receiving terminal. Offshore tank storage may either be floating or settled on the seafloor.
No commercially proven technology exists that allows LNG transfer in harsh open ocean conditions between a floating transport vessel such as a standard LNG Carrier (LNGC) and a floating storage vessel such as a Floating Liquefied Natural Gas (FLNG) vessel or a Floating Storage & Regas Unit (FSRU). An FSRU is a fixed asset near a market site, for storing LNG and converting the LNG to a vapor for delivery to on-shore facilities. An FLNG is a Floating LNG production vessel. It is a fixed asset at a production field, and converts natural gas to LNG and then stores it until it can be loaded onto an LNGC for delivery to market. For floating storage vessels to become technically and commercially viable, a reliable LNG transfer system is necessary, one that provides a high LNGC berth availability to ensure that gas delivery commitments can be met.
Conventionally, LNG transfer to/from a floating transport vessel is accomplished thru articulated hard-pipe loading arms, which require a relatively benign, sheltered location for the LNG transfer system to work. Virtually all operational LNG terminals are located onshore, in harbors or in waters that are sheltered from harsh ocean conditions. Requiring protected LNG transfer sites limits the number of potential sites for new terminals, and in many regions a suitable site simply is not available. On the other hand, public pressure is forcing LNG transfer facilities increasingly further offshore. For the US West Coast, few shallow water sites are available and the Pacific Ocean Meteorological & Oceanographic (metocean) conditions (sea states, currents & winds) complicate the problem and further limit the number of potential solutions.
Applying articulated loading arm technology in an open ocean location has been contemplated by many LNG Terminal Projects. In shallow water locations with milder metocean conditions, a gravity based structure (GBS) is a technically feasible solution. It basically serves as a breakwater, thus allowing loading arms to be used in a side-by-side berthing layout.
In deeper water applications, a floating storage vessel that is single point moored (SPM) allows the vessel to weathervane into the dominant metocean conditions, thus generally minimizing floating storage vessel motions. Loading arms have been proposed for side-by-side berthing arrangements. However, being able to predict the relative motions between the floating storage vessel and the floating transport vessel with the necessary high degree of certainty has proven to be difficult. Unlike a GBS, a floating storage vessel does not serve as a breakwater, and the tug boat operational problems are further compounded by the berthing approach layout. Additional concerns include damage to the floating transport vessel and floating storage vessel due to high relative motions of the vessels, and floating transport vessel breakout due to high loads on the mooring lines. All these issues combine to produce significant concerns over vessel berth availability of the various proposed offshore LNG terminal concepts, and thus exacerbate concerns with being able to meet gas delivery commitments.
Development work to date on new offshore LNG transfer systems has centered on aerial (in-air) systems, and recently work has started on floating hose systems. It has been found that these systems require the use of dedicated floating transport vessels and the resulting systems are overly complicated, often utilize too much new technology and are considerably expensive. Other ‘in-water’ bottom founded systems have been conceptualized, as well as a variety of platform based concepts, all of which utilize either loading arms or aerial hoses and have the same problems or concerns as stated above. Furthermore, most of these systems do not include LNG storage, which places them at a severe disadvantage.
More recently, the industry has renewed interest in floating hose based transfer systems and started development work. The appeal of a floating hose based system is that it mimics tandem ship-to-ship oil transfer systems, which are well understood and have a long, well proven history of safe, successful operation. Manufacturers have worked to develop a cryogenic floating hose based on a layered fabric (labyrinth) design with inner and outer wire reinforcements. Other designs utilize an insulated stainless “bellows-in-bellows” (BIB).
Several significant concerns exist for any floating hose system. By the nature of the design the hose floats at or near the high energy zone at the water surface. Thus the hose will experience significant wave loadings, movement and fatigue cycles, particularly if left in the water between LNG liftings. Hose manufacturers have only recently begun work to explore ways to retrieve/deploy the hose between liftings, but difficulties remain with all of these concepts. The typical manufacturing processes limit the hose sections to between 10 and 50 meters in length, perhaps 100 meters at the most. This necessitates that several flanged connections be incorporated into the hose string, which act as stiff points (stress risers). The flanged connections must further be insulated and sealed from heat and seawater ingress. A typical transfer system will include 2 to 3 LNG delivery hose strings, and possibly 1 vapor return hose string. Significant concerns exist over hose clashing, which is exacerbated by having several flanged connection in each hose string. Physical separation of the hose strings has been considered, but this limits hose movement which induces additional hard points and raises additional fatigue concerns. Finally, the means of lifting the hoses out of the water and connecting them to the floating transport vessel manifold is problematic and has yet to be defined.
EP1462358 describes an apparatus for mooring a tanker for transporting liquid natural gas. The apparatus is described as including a semi-submersible floating dock, a single point mooring system, and at least one rigid arm. Fluids from the ship are passed to the single point mooring system through flexible hoses. The flexible hoses are said to take a catenary form, and are further said to be either held above the water or partially in contact with the water.
U.S. Pat. No. 6,915,753 describes a similar assembly for passing fluid from one floating vessel to another.
U.S. Pat. No. 6,923,225 describes articulated hard-pipe loading arms for transferring liquid natural gas between a tanker vessel and a processing vessel.
U.S. Pat. No. 4,718,459 describes an underwater cryogenic pipeline system for transporting liquefied natural gas in underwater locations between an onshore production or storage facility and an offshore vessel.
US2004/0011424 teaches a system for transferring a fluid product between a carrying vessel and a fixed installation. A tubular conveying arrangement is described, comprising a connection device and a flexible transfer pipe connected to the installation. The free end of the flexible transfer tube is provided with handling means to move the free end between a connection position to the connection device and a disengaged storage position.
However, the current methods for transferring LNG in the open ocean have limited effectiveness. Aerial designs require dedicated floating transport vessels that are significantly modified versions of standard carriers, and are overly complicated and expensive. Floating hose systems, which lie in the wave zone, are subject to severe fatigue cycling as well as mechanical damage and abrasion from contact between hose strings. What is required is a LNG transfer system that provides safe operation, high berth availability, universal applicability, regardless of ship design, and conduit handling methods to substantially reduce conduit damage for offshore LNG transfer between floating vessels.